I. Field of the Invention
This invention relates to an improved system of soldering and in particular to an improved technique of depositing solder onto solder wettable contact pads with a substantially uniform amount on each pad.
II. Description of the Prior Art
Solder distribution onto mounting pads for surface mount boards has in the semiconductor industry generally been accomplished utilizing a screening process. In this technique, art work and screens must be fabricated having the solder deposition pattern. Then, a precision alignment process is carried out wherein the solder is screened onto the surface mount pads. The solder paste used for the process requires substantially long cure and bake times. Thus, in addition to the complexity of the alignment process, this prior art technique is relatively time consuming.
The prior art technique utilizing screening is further complicated by a requirement that the pattern mixes very fine lead pitch and width surface mount pads along with standard surface mount parts. For example, in the case of tape automated bonding the pitches vary from 4 to 20 mils while the standard surface mount parts have pitches in the range of 20 to 50 mils. Thus, the process requires that different amounts of solder be distributed on various parts of the board. Given the precision required, it is common to utilize separate screening steps, one for very fine lead pitch and width surface mount parts and second for the standard surface mount parts. There is the possibility of damaging the solder deposited in a previous step when multiple screening operations are carried out. Additionally, screening fine line solder presents a problem because the solder paste tends to stick in the openings of the screen as the openings get progressively narrower.
Another problem inherent in prior art screening systems is the difficulty of performing rework. Once a defective part has been removed there is no currently available technique to replace the solder on the board site. The solder which remains on the pads would be variable in thickness since there is no way to control the amount which remains with each lead of the part as it is being removed. While techniques exist to totally remove the solder after the part has been removed, replenishing by screening an isolated site is not yet feasible.
The prior art is also replete with a variety of techniques to deposit solder across the surface of a printed circuit. Typical techniques are so-called dip-soldering and wave-soldering. Wave-soldering involves pumping a molten solder through a nozzle to form a standing wave. In this process the entire side of an assembly containing printed conductors with the leads from the circuit components projecting through various points generally travels at a predetermined rate of speed over the standing surface of the wave of molten solder. The lower surface of the assembly is placed into contact with the upper fluid surface of the wave. By this technique, the solder wave in the first instance wets the joining surfaces and promotes through-hole penetration. This in turn helps to assure the formation of reliable solder joints and fillets. Wave soldering is illustrated in U.S. Pat. Nos. 3,705,457 and 4,360.144. An example of an immersion technique is illustrated in U.S. Pat. No. 4,608,941 wherein panels are immersed in a liquid solder bath and then conveyed to an air knife which levels the molten solder on the panels. The air knife is therefore used to effectively clear the panels of excess solder and only the printed patterns retain the solder.
Another example of a solder leveler is illustrated in U.S. Pat. No. 4,619,841. The technique is used in conjunction with dip-soldering techniques. Other techniques of selective deposition of solder onto printed circuit patterns are described in U.S. Pat. Nos. 4,206,254: 4,389,771; and 4,493,856.
U.S. Pat. No. 3,661,638 is also directed to a system for leveling and controlling the thickness of a conductive material on the walls of through-holes of a printed circuit board. The technique, for removing the excess amount of conductive material employs heating to melt a conductive material after it has been deposited and then, while the conductive material is in the plastic state, gyrating the board to cause the plastic material to move circumferentially about the through-hole and flow axially through the through-hole.